<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button>删除</button>
    <ul>
        <li>1</li>
        <li>2</li>
        <li>3</li>
    </ul>
    <script>
        var ul=document.querySelector('ul')
        var btn=document.querySelector('button')
        btn.onclick=function(){
           if(ul.children.length==0){
               this.disabled=true
           }else{
               ul.removeChild(ul.children[0])
           }
        }
        // ul.removeChild(ul.children[0])
    </script>
</body>
</html>